The purpose of this study will be to identify faceseal leak sites which occur during the wearing of half-mask, air-purifying respirators. It is also intended to determine if these leak sites are associated with any critical anthropometric facial dimensions of the test subjects. This will be accomplished by having a test subject wear a respirator in an exposure chamber containing an aerosol of a fluorescent whitening agent which will leave a visible trace of penetration when illuminated by longwave ultraviolet light. A quantitative fit test will be conducted while the subject is in the chamber to monitor aerosol concentration and penetration into the respirator. Aerosol deposition at leak sites will be documented by photographic methods. It is hypothesized that most leaks will occur around the area of the nose and will be over a relatively large area as opposed to being point leaks. A prototype aerosol generating, conditioning and exposure system has been developed to conduct these tests. The aerosol of the fluorescent whitening agent has been characterized as to size distribution and conforms to that recommended for fit testing. The system has been used to perform preliminary tests on mannequins using stimulated breathing and artificial leaks. Photographs under ultraviolet lights of the mannequins have illustrated the imposed leaks as well as other faceseal leaks. Results of this study would have both practical and theoretical applications. Data from this study may contribute to more accurate theoretical modeling of respirator leakage by indicating leak location and geometry. Both of these factors are critical in defining flow regimes, yet are the subject of assumptions in the prominent leak models currently used. It may also provide information which could improve the design and fit of respirators. It has been estimated that over 10 million of these devices are used for protection by workers in the U.S. each year.